The present invention relates to fluid coupling devices, such as viscous drives and hydraulic fluid drives. The fluid coupling devices are generally of the type that include both a fluid operating chamber and a fluid reservoir chamber, and in addition valving to control the quantity of fluid in the operating chamber.
A fluid coupling device may be referred to as a viscous drive, a hydraulic fluid drive, or a combination thereof. A viscous drive generally refers to a fluid coupling device that has clutch members that are engaged due to the amount of friction therebetween. A hydraulic fluid drive generally refers to a fluid coupling device that is engaged via hydraulic fluid and/or pressure thereof.
Viscous and hydraulic drives have become popular due to their ability to cycle repeat, engage at higher engine speeds, and have varying degrees of engagement. Hydraulic fluid drives also allow for full engagement and thus increased operating speeds. Viscous and hydraulic drives may be operated using an open loop fan drive control methodology. Open loop fan drive control allows for the speed of a fan drive to be adjusted continuously to any fan speed in response to changing situations. Open loop control is particularly advantageous in viscous and hydraulic drive systems, which can have infinite variability.
The use of such a methodology, however, can cause an over-speed condition to arise in certain situations. For example, when a vehicle is traveling down a steep hill the engine speed of the vehicle can increase and thus cause the fan speed to increase, due to fan load on and coupling of the fan to the engine. The fan in essence is performing as a brake on the engine. Energy is transferred from the drivetrain and engine to the fan. The increase in engine speed can exceed beyond a normal operating speed, and be such to cause the fan to spin at high speeds not originally designed. In other words, the fan can “race” or spin at such a speed as to cause degradation to fan drive system internal components. This condition especially arises when the fan drive system is fully engaged and there is negligible slip between the engine and the fan.
Thus, there exists a need for an improved fan drive system that prevents such an over-speed condition from arising.